Add atonum() - Convert string to natural number, returns -1 on error.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

README.md

@@ -13,6 +13,11 @@ traversing lists while deleting/freeing nodes.
 Helper Macros
 -------------
 
+- `atonum(str)`
+
+  Convert string to natural number, works for 32-bit non-negative
+  integers.  Returns -1 on error.
+
 - `blkdev(dev)`
 
 Create block device

lite.h

@@ -101,6 +101,20 @@ static inline int fisslashdir(char *dir)
                          return 0;
 }
 
+/* Convert string to natural number (0-2147483647), returns -1 on error. */
+static inline int atonum(const char *str)
+{
+	int val = -1;
+	char *errstr;
+
+	if (str) {
+		val = strtonum(str, 0, INT32_MAX, &errstr);
+		if (errstr)
+			return -1;
+	}
+
+	return val;
+}
 
 /* Validate string, non NULL and not zero length */
 static inline int string_valid (const char *s)